The invention relates in general to speech signal processing and in particular to equalization of a speech signal in a mobile phone in order to improve the intelligibility of transmitted speech.
It is a widely known and used method to improve the auditory effect of sound, and of speech, which is a special case of sound, with an equalizer. The frequency spectrum of a sound can be equalized, whereby the frequency response peaks or pits corresponding to certain frequencies are equalized closer to the average level, or certain frequencies or frequency areas can purposely be amplified or attenuated so that they deviate from the mean level. In addition to improving the auditory effect, equalization can also help people suffering from certain types of hearing defects discern sounds better.
Equalization can be performed electrically or mechanically. Electrical equalization requires the use of various filter circuits in the device that processes sound. Known solutions in the field of analog signal processing include passive RC filters, active filters based on operational amplifiers and special equalizer chips, and in the digital field, digital signal processors which can be programmed to realize very complex filter arrangements. Mechanical equalization is performed by selecting the mechanical properties of the loudspeaker reproducing sound and of its immediate vicinity such that a desired frequency response is achieved.
In mobile phones, equalization is needed to improve the quality and intelligibility of the sound that travels from the loudspeaker to the user's ear. The sound from the loudspeaker seems distorted because usually the loudspeaker does not reproduce all frequencies in the same way and because there occurs an acoustic leakage between the loudspeaker and the user's ear which affects different frequencies in different ways. FIG. 1 is a graph illustrating two frequency response measurements: in the measurement arrangement corresponding to the upper curve the loudspeaker and the instrument corresponding to an ear are protected against acoustic leakage (strong acoustic coupling) and in the arrangement corresponding to the lower curve the acoustic leakage corresponds to normal usage of a mobile phone. The figure shows clearly that as the acoustic coupling gets weaker because of the leakage, especially the reproduction of low frequencies (&lt;1 kHz) gets worse.
A known method which has been used in attempting to equalize the distortion of sound is the use of special loudspeakers applying acoustic feedback. The method is of a mechanical nature, because in it an attempt is made to build the loudspeaker such that its mechanical properties compensate for the attenuation of low frequencies. An air gap is left between the loudspeaker and its mounting body, whereby the width of the air gap is a significant factor as regards acoustic feedback. To achieve correct dimensioning and operation, strict mechanical tolerances must be followed in the assembly of the loudspeaker, which increases manufacturing costs. The frequency response of the mechanical arrangement cannot be changed later, which makes the arrangement inflexible. Furthermore, the method cannot be applied in the implementation of a hands-free loudspeaker which is a widely used option in mobile phones.
By suitable design of the electromechanical properties, a loudspeaker can be realized which by nature amplifies low-frequency signals. Also this arrangement is inflexible, since the frequency response is determined in the manufacturing process of the loudspeaker and it cannot be changed e.g. with an electrical control signal.
Electrical filter solutions can usually affect the frequency response of a sound reproduction arrangement in a manner more versatile and flexible than can mechanical or electromechanical solutions. In a mobile phone, however, space, power consumption, computing capacity and printed circuit board area are very critical factors for which the electrical filter should have as low a demand as possible. Both passive and active analog filters increase component costs and consume electrical power as well as printed circuit board area. If one wishes to employ these to build an adjustable equalizer, the operation of which can be changed with a control signal according to need, the arrangement easily becomes complex and expensive to manufacture. In addition, analog filters often have a certain, rather limited, operation range, e.g. a fixed center frequency or insufficient amplification capacity.
Digital filter arrangements, which in principle can be realized as separate digital circuits but which advantageously are programmed into processes executed by a digital signal processor, have proven superior to their analog counterparts in many respects. Since most popular mobile phone constructions already include a digital signal processor, or DSP, the programmable filter arrangement does not significantly increase the number of components in a mobile phone. At the same time, power consumption and space requirements are kept within reasonable limits. In addition, the operation of processes programmed in a DSP can be easily changed in a desired way by bringing certain control signals to the processor.
However, prior art DSP-based equalizer arrangements in mobile phones do not function in an optimal manner in all situations. It is known that since mobile phones are used in a wide range of conditions, in which e.g. the ambient noise level both in the talking and in the listening environment varies, reproduction of speech in the best possible manner is impossible if equalization is always performed in the same way. In addition to the environmental conditions, the use of various auxiliaries, such as a hands-free loudspeaker, earphone loudspeaker or an additional loudspeaker, affects the operation required of the equalizer.